Permanent magnet motors such as a brushless DC (Direct Current) motor have been driven sinusoidally using a low cost 8 bit microcomputer. One example of such technology is disclosed in JP 2000-324873 A, hereinafter referred to as patent publication 1, in which rotor positioning is sensed by a position sensor configured by a Hall element, for instance, whereafter sinusoidal wave data is read from medium such as a ROM (Read Only Memory) based on the sensed rotor position and motor winding is energized based on the read data. Under such configuration, phase control can be readily executed by modifying the address of the ROM from which sinusoidal wave data is to be read.
However, since position signal sensed by the position sensor is coarse, having an electrical angular interval of 60 degrees, time lag is frequently produced to the actual time of operation due to the read time expended in reading the data stored in the ROM. This often leads to failure in outputting a drive signal pattern corresponding to the proper position.
JP 2004-328821 A hereinafter referred to as patent publication 2 attempts to eliminate such time lag by comparing the position detection signal with the position command signal and resetting the position command signal if a deficit equal to or greater than a predetermined value exists between the two, to synchronize the motor position and the drive position.
In the approach disclosed in patent publication 2, a sudden change in voltage is observed from time to time when comparing the drive waveform before and after the synchronization. This is caused by the dramatic change in the read address originating from the correction of shift in the position signal and unwantedly leads to distortion in the sinusoidal wave representing the motor drive waveform which ultimately produces noise. Such noise emission will significantly impair product quality in automobile motor drive systems which quietness is a key factor in product evaluation.